The development of the respiratory system is continuous from the fourth week, when the respiratory diverticulum appears, to term. The 24‐week potential viability of a foetus (approximately 50% chance of survival) is partly because at this stage the lungs have developed enough to oxygenate the blood. Limiters to oxygenation include the surface area available to gaseous exchange, the vascularisation of those tissues of gaseous exchange and the action of surfactant in reducing the surface tension of fluids within the lungs.
Development of the respiratory system includes not only the lungs, but also the conducting pathways, including the trachea, bronchi and bronchioles. Lung development can be described in five stages: embryonic, pseudoglandular, canalicular, saccular and alveolar.
Although not in use as gas exchange organs in utero, the lungs have a role in the production of some amniotic fluid.
The development of the respiratory system begins with the growth of an endodermal bud from the ventral wall of the developing gut tube in the fourth week (Figure 32.1).
To separate the lung bud from the gut tube two longitudinal folds form in the early tube of the foregut, meet and fuse, creating the tracheoesophageal septum. This division splits the dorsal foregut (oesophagus) from the ventral lung bud (larynx, trachea hese structures remain in communication superiorly hrough the laryngeal orifice.
Being derived from the gut the epithelial lining is endodermal in origin, but as the bud grows into the surrounding mesoderm reciprocal interactions between the germ layers occur. The mesoderm develops into the cartilage and smooth muscle of the respiratory conduction pathways.
In the fifth week the tracheal bud splits and forms two lateral out-growths: the bronchial buds. It is at this early stage we see the asymmetry of the lungs appear; the right bud forms three bronchi and the left two. The bronchial buds branch and extend, forming the respiratory tree of the three right lobes and two left lobes of the lungs (Figure 32.1).
Up to week 5 the first period of lung development is known as the embryonic stage.
From 6 weeks their development enters the pseudoglandular stage. The respiratory tree continues to lengthen and divide with 16 20 generations of divisions by the end of this stage (Figure 32.2). Histologically, the lungs resemble a gland at this stage.
Epithelial cells of the bronchial tree become ciliated and the beginnings of respiratory elements appear. Cartilage and smooth muscle cells appear in the walls of the bronchi. Lung‐specific type II alveolar cells (pneumocytes) begin to appear. These are the cells that will produce surfactant.
The pseudoglandular stage ends at approximately 16 weeks, by which time the entire respiratory tree, including terminal bronchioles, has formed (Figure 32.2).
During the next phase, known as the canalicular stage (17–24 weeks), the respiratory parts of the lungs develop. Canaliculi (canals or tubes) branch out from the terminal bronchioles. Each forms an acinus comprising the terminal bronchiole, an alveolar duct and a terminal sac (Figure 32.2). This is the primitive alveolus.
The duct lumens become wider and the epithelial cells of some of the primitive alveoli flatten to form type I alveolar cells (also known as type I pneumocytes, or squamous alveolar cells). These will be the cells of gaseous exchange.
An invasion of capillaries into the mesenchyme surrounding the primitive alveoli brings blood vessels to the type I alveolar cells. Towards the end of the canalicular stage some primitive alveoli are sufficiently developed and vascularised to allow gaseous exchange, and a foetus born at this stage may survive with intensive care support.
The saccular stage (or terminal sac period, from 25 weeks to birth), describes the continued development of the respiratory parts of the lungs. Type II alveolar cells (also known as type II pneumocytes, great alveolar cells or septal cells) begin to produce surfactant, a phospholipoprotein that reduces the surface tension of the fluid in the lungs and will prevent collapse of the alveoli upon expiration and improve lung compliance after birth.
During this stage many more primitive alveolar sacs develop from the terminal bronchioles and alveolar ducts. The blood air barrier between the epithelial type I alveolar cells and endothelial cells of the capillaries develops in earnest, and the surface area available to gaseous exchange begins to increase considerably.
The final alveolar stage (36 weeks onwards) begins a few weeks before birth and continues postnatally through childhood. Alveoli increase in number and diameter enlarging the surface area avail- able to gas exchange (Figure 32.2). The squamous (type I alveolar) epithelial cells lining the primitive alveoli continue to thin before birth, forming mature alveoli (Figure 32.3). Septation divides the alveoli. Surfactant is produced in sufficient quantities for normal lung function with birth. Continued development through child-hood will increase the number of alveoli from 20–50 million at birth to around 400 million in the adult lung (Table 32.1).
Two classes of blood circulation are present in the lungs: pulmonary and bronchial. Pulmonary arteries derive from the artery of the sixth pharyngeal arch and accompany the bronchial tree as it branches, while the pulmonary veins lie more peripherally. This part of the circulatory system is involved in gaseous exchange, and until birth little blood flows through the pulmonary vessels. For the changes to this circulatory system that occur at birth see Chapter 31.
Bronchial vessels supply the tissues of the lung. These vessels are initially direct branches from the paired dorsal aortae.
Respiratory distress syndrome (hyaline membrane disease) caused by a lack of surfactant results in atelectasis (lung collapse). This affects premature infants, and treatment options include a dose of steroids given to the infant to stimulate surfactant production, or surfactant therapy. Surfactant is administered to the infant directly down a tracheal tube. These treatments together with oxygen therapy and the application of a continuous positive airway pressure using a mechanical ventilator mean that the prognosis is good in many cases.
Oesophageal atresia and tracheoeosphageal fistulas are relatively common abnormalities. If the separation of the trachea from the foregut is incomplete various types of communicating passages may persist. This type of abnormality is often associated with other faults, including cardiac defects, limb defects and anal atresia. It is also possible that an oesophageal atresia will lead to polyhydramnios as the amniotic fluid is not swallowed by the foetus, or pneumonia after birth as fluid may enter the trachea through the fistula. Surgery is generally required.
Ectopic lung lobes and abnormalities in the branching of the bronchial tree rarely produce symptoms.
Congenital cysts of the lung common infection sites and difficulty in breathing.